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Chemists from the National University of Singapore (NUS) have developed innovative materials that can harness both singlet and triplet excited states for efficient, metal-free photocatalysis.

Photocatalysis is a promising green technology that uses sunlight to drive chemical reactions. However, most existing materials exploit only one type of light energy—either the singlet or triplet excited state. This limits their efficiency and range of applications, especially under natural sunlight, which contains a broad spectrum of wavelengths.

A research team led by Professor Donglin Jiang from the Department of Chemistry at NUS has developed a new class of covalent organic frameworks (COFs) that can simultaneously use both singlet and triplet excited states. These dual-channel COFs are designed with donor and acceptor units arranged in ordered columns, enhanced by precise hydrogen bonding for improved stability and performance under light exposure.

The research findings have been in the journal Nature Materials.

This breakthrough allows for highly efficient photocatalytic reactions powered by red and near-infrared light, which are regions often underutilized by existing catalysts. The metal-free nature of these materials makes them environmentally friendly and cost-effective, with potential uses in clean fuel production, pharmaceuticals, and green industrial processes.

The COFs designed by the research team feature alternating donor and acceptor molecules that are stacked in a well-aligned columnar structure. This unique design enables fast charge and energy transfer across the framework and allows the activation of two distinct types of excited states:

• Singlet states, which are ideal for fast and selective chemical reactions.
• Triplet states, which last longer and can drive more challenging transformations.

Few materials can access both pathways effectively, but these new COFs manage it in one unified system, allowing more sunlight to be harnessed for catalytic purposes. Tests showed that the materials perform exceptionally well even under low-energy red light, achieving some of the highest turnover frequencies reported for metal-free systems. They also do not require any added metals or chemicals to boost their performance.

In one example, under red light (620 nm), the material Hâ‚‚P-BT(OMe)â‚‚-COF converted 98% of a chemical called benzylamine into the desired product in 10 minutes. It also demonstrated high efficiency, with a turnover frequency of 1,298 per hour, meaning each active site in the material can carry out over a thousand reactions every hour.

The frameworks are also modular and can be customized for different types of reactions by simply changing the donor or acceptor units. Their porous structure supports efficient reactant transport, making them ideal for industrial applications such as continuous-flow processes.

Professor Jiang said, "This work opens a new avenue for sustainable photocatalysis. Our dual-channel COFs demonstrate that it is possible to harvest light more completely and efficiently using metal-free materials. We are excited to explore how far we can take this technology in real-world applications."

The research team plans to tailor these COFs for use in large-scale applications, such as solar fuel generation and environmental remediation. By tuning their structure, the materials could be adapted for use in a wide variety of light-driven chemical processes.